html,
body,
p,
ul,
li {
  padding: 0;
  margin: 0;
}
header {
  width: 100%;
  height: 80px;
  background-color: red;
}
#content {
  width: 800px;
  height: 400px;
  border: 1px solid gray;
  margin: 100px auto 0;
  position: relative;
  overflow: hidden;
}
#content div {
  transition: 0.5s;
}
